(1) Field of the Invention
The present invention relates to a butterfly valve which can maintain water-tightness by tightly depressing a disk on a valve seat in accordance with an operation of a cam.
(2) Description of Related Art
A conventional butterfly valve comprises a cylindrical body, a metallic valve seat fixed on an inner circumferential of the body, a disk rotating about a rotating shaft to open and close a conduit, and a seat-ring mounted around the disk.
However, in the conventional butterfly valve, since the leak is prevented by contacting force of the disk on the valve seat, if the seat is worn by repeated opening and closing operations, the leak occurs. In addition, since the disk is supported by only the rotating shaft, it is subject to a severe shock by reversed water pressure, deteriorating the water-tightness. Particularly, the valve having a valve seat made of rubber material cannot be used with corrosive liquids because of the valve seat degradation. A metallic valve seat is not easy in its manufacture because a high preciseness is required to make the metallic valve seat.
To solve the above described problems, a valve using hydraulic cylinders which push the disk toward the valve seat has been proposed. However, this makes the structure of the valve complicated because additional components for operating the hydraulic cylinders are required, resulting of a high cost in manufacture.